A light transmission type smoke detector gives a prompt signal at the occurrence of a fire by detecting attenuation of light beam due to smoke coming in a light path provided therein. In light transmission type smoke detectors of the prior art, light attenuation due to smoke existing in the light path and apparent attenuation in light intensity due to aging of the light source, due to degradation in sensitivity of the light-receiving element or due to contamination of lenses and reflecting mirrors in the light path caused by dust or anything else are discriminated by measuring only the degree of light attenuation per time, and thereby prevention of erroneous alarming is intended. In other words, an abrupt change in light intensity caused by smoke generated by a fire and a slow change in light intensity caused by the above mentioned various factors are discriminated only in accordance with light attenuation differentiated by time.
Therefore, when the smoke concentration very slowly increases as in the case of occurrence of a smouldering fire, it is difficult for them to sense absolute smoke concentration and to distinguish the light attenuation due to smouldering from the light attenuation due to other conditions such as referred to above, since there is little or no difference in the light attenuation per unit time therebetween. Therefore, such apparatuses often fail to detect a real fire.
I now have invented a novel light transmission type smoke detector in which the above-mentioned defect is diminished.